Recycling of input for comparative analysis
- input for a filter is retained until a new specification
is called for. This allows various IIR designs to be
compared with FIR window designs as well as FIR equiripple
designs.

Use of 64-bit floating point for all design
calculations ensures that maximum accuracy is maintained for
the design calculations.

Specification file which allows the
retention and retrieval of filter specifications.

Help screens for all data entry fields.

Cursor tracking on all frequency domain
plots. X and Y coordinates read out automatically as the
cursor is tracked on the function by holding the mousebutton
down and dragging across the window as desired.

Graphical Zooming is available.

Design
Methods：

For the IIR design, a
normalized Lowpass Analog Transfer Function is generated based
on the given filter specifications. This normalized analog
transfer function is transformed via the analog transform
formulas with the values suitably chosen for prewarping. The
unnormalized transfer functions for Lowpass, Highpass, Bandpass
and Bandstop Filters are then transformed to the digital domain
via the bilinear transformation. Details of the transformations
are printed in the output file SFIL.OUT Output for each design
will be accumulated in this file until it is printed.

The filter characteristics including the
impulse response are simulated via cascaded second order
sections with the poles and zeros grouped using the L. B.
Jackson algorithm to minimize stability problems. For the FIR
design with windows, the number of taps in the filter are
determined using the Kaiser window calculation again based on
input specifications.

The coefficients for the taps are then
determined by using Fourier series design techniques for
computing the impulse response and the window coefficients. The
Equiripple FIR design uses the Remez exchange algorithm to
determine an optimal solution. It should be noted that this
algorithm can give erroneous results if the filter
specifications are too tight.